Delete Contact (GDPR purge)

Erase a person: the contact, every identifier that resolves to them, every per-agent caller record of them, and every memory ever extracted about them - on every agent and every channel, in one call. This is the right-to-be-forgotten path. `DELETE /v1/agents/callers/{caller_id}` erases one agent's record of someone and has to be fired once per agent and once per channel; this reaches all of them, because "everything about this person" is a question the contact model can answer. Conversations survive - they are the billing and forensic record - but the facts extracted from them, and the copies of those facts frozen into past prompts, do not. The identifiers are tombstoned as part of the purge, which is what stops the person's phone number ever resolving again: a later merge naming the purged contact moves rows that are already dead rather than bringing a link back to life. Idempotent: re-deleting an already-purged contact returns all zeros rather than a 404, so a client that never saw the first response can safely repeat it. Returns `200` with the row counts rather than a bodiless `204`: the counts are the audit evidence a privacy operator needs, and re-reading a corpus you just asked us to erase is not a way to obtain them. Same deliberate exception `DELETE /v1/agents/callers/{caller_id}` takes.